Damping of mechanical vibrations, as opposed to isolation of vibrations, may be desirable in some applications to reduce dynamic motions. Usually, damping is required in systems that are load controlled, where loads applied to the system cause dynamic oscillatory motions. Adding large amounts of hysteretic damping to these systems can reduce these resultant motions. One application where high damping may be required is in the so-called hingeless or flexbeam rotor system of a rotary wing aircraft, i.e., a helicopter. Typically, these composite rotor systems do not exhibit enough internal damping to damp lead-lag motions. As such, a lead-lag damper may be required to provide supplemental damping to the system. Prior art systems have greatly increased system damping by addition of passive dampers with the resulting reduction in rotor blade lead-lag motions.
One such passive elastomeric damper is taught in U.S. Pat. No. 4,778,343 to Hahn et al., the disclosure of which is hereby incorporated by reference herein. The Hahn '343 patent describes a system which utilizes dual elastomer dampers 9 to damp lead-lag motions. At the same time, these dampers also react to flapping loads. The dampers 9 are generally used in opposed pairs, one on the top of the torque tube and one to the bottom thereof. These passive dampers 9 are bonded in highly damped polymers (loss factors of 0.5 or more). The highly damped material hysteretically damps the motion of the blades by converting motion into heat.
U.S. Pat. No. 5,092,738 to Byrnes et al. which is hereby incorporated by reference herein, describes another passive damper system for a hingeless or flexbeam rotor system. The Byrnes et al. '738 patent teaches using a spherical elastomeric bearing 52 to perform a centering function, accommodate torsional and cocking motions, and react to the flapping loads. U.S. Pat. No. 4,893,988 to Sato teaches yet another passive damper system. A lead-lag damper for a more conventional helicopter rotor system is described in the commonly assigned U.S. Pat. No. 3,758,230 to Potter.
As an improvement over passive elastomer dampers, damper devices have been developed by the assignee of the present invention which utilize combinations of fluid and elastomer to increase damping levels and linearity achievable by passive elastomer dampers. The commonly assigned and copending U.S. application Ser. No. 07/934,402, which is incorporated by reference herein, describes a fluid and elastomer damper for use on an articulated rotor system. The device described in the abovementioned '402 application utilizes a fluid contained within the damper which is throttled back and forth between opposed chambers and through a fluid passageway and to create additional damping over and above what is available by elastomeric dampers (hysteresis) alone. These dampers will be referred to as throttled-type dampers. The device further includes means for limiting the dynamic pressure buildup in the damper.
These throttled-type dampers produce fluid damping in addition to the hysteretic damping which varies as a square of the velocity of the fluid flow taking place within the fluid passageway. However, although these throttled-type dampers provide excellent properties, a simpler construction may be needed in some applications. Furthermore, providing two fluid cavities can take up space envelope that may not be available in some applications. This is especially true in helicopter rotor applications where any increase in size of the damper will mean more exposure to the air stream, and thus, a dramatic increase in the drag on the rotor blade system. In addition, without proper sizing of the passages and viscosity, throttled-type dampers tend to be somewhat nonlinear. A fluid and elastomer damper for a more conventional helicopter rotor system is described in U.S. Pat. No. 4,566,677 to LePierres.
The prior art teaches of devices which utilize an inner member for moving through a fluid, as opposed to throttling, to provide additional nonhysteretic damping. U.S. Pat. Nos. 3,874,646 to Vernier, 3,154,273 to Paulsen, and 3,141,523 to Dickie, describe devices whereby a piston is used in a closed cavity which includes a viscous material. Movement of the piston within the cavity causes a stirring of the fluid, thus increasing damping over and above that available from the elastomer alone. However, these types of devices lack the ability of providing very high damping forces and at the same time exhibiting the ability to carry large loads. Further, the damping forces available have not been optimized.
U.S. Pat. No. 4,790,521 to Ide et al., which is hereby incorporated by reference herein, describes a fluid mounting including a resistance plate member 10 to stir a viscous fluid 11 causing a resistance force. The Ide at al. '521 device, however, lacks the capability of carrying high axial or radial loads. Further, the resistance plate member 10 does not act at the most advantageous point within the mounting. Near the base of the resistance plate member there is no relative movement between it and the fluid, therefore no damping is created. Further yet, the use of the viscous shear component in inefficient.
U.S. Pat. Nos. 4,927,122 to Brumrne et al., 4,817,926 to Schwerdt, 4,811,919 to Jones, 4,779,585 to Behrens et al., 4,770,396 to Jouade, 4,741,520 to Bellamy et al., 4,607,828 to Bodin et al., 4,236,607 to Halwes et al., 3,888,449 to Jablonski et al., 3,167,157 to Thorn, 3,107,752 to McLean, and 2,919,883 to Murphy, disclose various piston designs and concepts.